Set user agent span attribute on standalone spans

[mydea]

While investigating #11646, we found the root of the problem:

INP ingestion depends on the user agent (because it differs by browser). When using a tunnel the user-agent may often not be forwarded, leading to INP not being measured correctly.

We should fix this server-side by not ignoring INP spans without a user agent, but we should also add the user agent as an attribute to these spans so they can be used (if set) to improve handling.

Based on https://opentelemetry.io/docs/specs/semconv/attributes-registry/user-agent/, we should set user_agent.original to the full user agent string.

[cleptric]

@jjbayer @edwardgou-sentry we tested if setting event.request.headers: {'user-agent': '...'} on the INP span would do the trick, but it looks like it does not. In any case, I would propose we move forward with user_agent.original inside span.data. Sounds good to you?

[edwardgou-sentry]

@jjbayer @edwardgou-sentry we tested if setting event.request.headers: {'user-agent': '...'} on the INP span would do the trick, but it looks like it does not.

Did you test on an AM3 plan org? There was another issue with standalone span ingestion on AM3 plans that we just found and fixed today: https://github.com/getsentry/getsentry/pull/14570

In any case, I would propose we move forward with user_agent.original inside span.data. Sounds good to you?

👍 sounds good to me!

[Lms24]

As discussed yesterday with @jjbayer I opened getsentry/relay#3815 to track the Relay changes.

[reyoucat]

👍

[jjbayer]

@Lms24 @mydea for events, we also try to extract the browser name from client hints, should we do the same for spans? Any idea if there's an OTel convention for client hints attributes?

[mydea]

@jjbayer there is https://opentelemetry.io/docs/specs/semconv/resource/browser/ which specifies some stuff that should be taken from the client hints, is that would you're thinking about?

[Lms24]

Update: I tested this change with a NextJS up using tunnelRoute and I can confirm that INP values show up in the Sentry UI. Thanks to everyone involved for fixing this!